There is a misconception among practitioners that corneal topographic maps are highly diagnostic on their own. In many circumstances a map is highly informative toward making a diagnosis, and in some cases is definitive.

I urge caution, however, in accepting topographic maps on blind faith as the be all and end all in diagnosing corneal disorders. To illustrate how you can be steered down the wrong path by topographic maps, this and my next column or two will present some masquerades for you to consider.

Figure 1. Topography of a patient with an oblate bowtie pattern resulting from
PRK.

What You See

Figure 1 reveals an eye with a central pattern that is flatter than the periphery, with an arcuate inferior region of
steepening. Figure 2 also reveals a relatively flat central region with an inferior arcuate region of
steepening. Do these two maps show the same condition?

What You Get

No, they do not. Figure 1 shows an oblate bowtie pattern generated from a photorefractive keratectomy
(PRK) procedure. The apparent inferior steepening is actually the mid-peripheral steepening knee found at the edge of the treatment zone, particularly in older procedures. The ablation is decentered superiorly, giving this inferior steepening appearance.

The eye in Figure 2 demonstrates the classic appearance of pellucid marginal degeneration
(PMD), a corneal thinning disorder that produces mid-peripheral thinning and steepening (it may be the same disease as
keratoconus, but located peripherally rather than more centrally). The topography map demonstrates an against-the-rule central pattern with a pincer-like pattern of warmer colors wrapping its arms around the center of the map. The marked inferior steepening is consistent with the steepening found superior to the arcuate thinned region seen in
PMD.

These two cases represent similar corneal topography maps arising from very different circumstances. A detailed case history and careful slit lamp examination would have provided substantial knowledge to aid in the proper diagnosis. However, as I mentioned at the start of this column, depending exclusively on the topography maps could easily lead you astray and potentially result in confusion or an incorrect diagnosis.

Dr. McMahon is a professor and Director of the Contact Lens Service at the University of Illinois at Chicago Dept. of Ophthalmology & Visual
Sciences.